Wondering if my Thermostat has gone in the car?

Ok, here's my question.

When running the temp in the car shoots right up to red when the engine warms up, there is no smell of over heating or any signs of it (But not sure whether it is over heating) when I rev the engine or just generally drive the engine temp comes back down and sits at half way all nice. My radiator fan boots in when the temp hits half way, and both radiator hoses get warm once the engine has warmed up. But typically as I'm sure you will have guessed, the heaters have packed up and will only blow cold air regardless of the setting. Now this would be fine, but living in England I have to de-mist the windows even in the summer

Wanting to get this fixed, I'm hoping it is the thermostat, as it's the cheapest option, I've herd it could also be an air-lock. I have bled the radiator system a couple of times too.

Was also told, worse case scenario it will be the heater matrix?

Any sort of advice would be smashing on this one please ladies and gents.

Normally a broken thermostat would make the car take ages to warm up(on the gauge).
I did have a similar issue once while driving on a motorway the gauge went into the red without warning but when i slapped the dash above the instruments it went back down to normal and hasn't done it since.!?!?!?

Ok, update on this one.

Let the engine warm up (slightly, it was raining heavy so gave up waiting) with the radiator cap off, the water in the radiator dissapeared, so I filled it back up. Noticed upon revving the heaters would now get warm, and then when idling the heat would drop.

Now I think it's just an air lock now, when it stops raining I'm going to bleed the system and see if that cures it. However I've had to do this a few times over the space of the year, does anyone else think the engine gaskets might have gone?

Doesn't necessarily have to be the gaskets, could be a small leak somewhere else. If the engine is running normally it's most likely not the gaskets. Is there water in the oil?

Quite possibly a pin hole in a rubber pipe that only opens when the car gets hot.

Cheers

D